In television signal processing, various signal processing techniques are preferably changed in accordance with changes in the signal-to-noise conditions. For example, peaking of the higher frequency portions of the luminance signal should be lowered or discontinued for a noisy signal. As a further example, it is desirable to reduce color saturation for a noisy signal, thereby to avoid the kinescope being driven into over-saturation or "blooming" by noise accompanying color-difference signals.
U.S. Pat. No. 5,113,262 issued May 12, 1992 to C. H. Strolle et alii, entitled "VIDEO SIGNAL RECORDING SYSTEM ENABLING LIMITED BANDWIDTH RECORDING AND PLAYBACK" describes a modification of the VHS video cassette recording system, in which a wideband luminance signal has its spectrum folded before being used to frequency-modulate the luminance carrier during the recording of the video tape cassette, and in which the folded-spectrum signal recovered during playback by demodulating the luminance carrier is unfolded to reproduce the wideband luminance signal. During the recording of noisy composite video signals it may be desirable to limit the bandwidth of the luminance signal and avoid spectrum folding procedures. During the playback of the noisy video signals recorded without folding the spectrum of the luminance signal, the unfolding procedures can be dispensed with responsive to sensing the noise being above a prescribed level.
Generally speaking, non-coherent, random noise (such as thermal noise) accompanying a video signal can be detected by monitoring that signal during portions of the vertical blanking interval where no video information is being transmitted. Such a concept is well described in the television literature. In a video cassette recorder, however, monitoring a video signal during vertical blanking intervals in order to measure accompanying noise tends to be impractical, because the nature of video cassette recorder signal processing frequently causes additional noise and distortion during the vertical interval. Alternatively, one may consider monitoring a portion of the television horizontal scanning line. Although monitoring of the noise superposed on horizontal synchronizing pulses is a possibility, synchronization compression is likely to occur in television and video cassette recorders, and as a result monitoring of the peak portions of the horizontal synchronizing pulses is not especially reliable for noise detection.
In U.S. Pat. No. 4,873,574 issued Oct. 10, 1989 and entitled "NOISE MEASUREMENT FOR VIDEO SIGNALS", T. A. Darby describes the monitoring of a video signal during backporch intervals as respectively follow horizontal synchronizing pulses, in order to measure the noise accompanying video signals over a field interval. One noise sample is taken during each backporch interval, and the samples are accumulated over a field interval to provide a measure of the noise content accompanying the field of video information.
However, in a variety of video signal processing environments, noise can vary within the duration of one field of video, giving rise to a need for noise to be detected and measured more frequently than once a field. One noise sample taken during each backporch interval does not provide a noise-signature component that can be relied on to be representative of the noise accompanying a video signal for each horizontal line, since the single sample may be taken during a time noise energy is atypical.